As mobile broadband data network continues its migration to all-Internet Protocol (IP), the Internet Engineering Task Force (IETF) protocols are replacing legacy Signaling System No. 7 (SS7) based protocols. Specifically, SCTP (Stream Control Transmission Protocol) has become the de facto transport layer for all control plane signaling. SCTP was designed to have features missing from other two common IP transport protocols such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). For example, SCTP supports multi-homing where it can bind to more than one IP address across different subnets. The multi-homing feature allows path resilience to the SCTP peers. It also helps with network interface failure on the peer machine.
In order to use SCTP for SS7 applications, various user adaptation layers were introduced such as SS7 Signaling Connection Control Protocol (SCCP)-User Adaptation Layer (SUA), Message Transfer Part (MTP) Level 3 User Adaptation Layer (M3UA), MTP Level 2 User Adaptation (M2UA), Integrated Services Digital Network (ISDN) User Adaptation (IUA), etc. which allow the use of a subset of SS7 protocol layers. These adaption layers have their own overhead but were necessary for the legacy applications that required SS7 as underlying serving protocol. As newer telecom protocols and applications take direct advantage of IETF based protocols and as such they use SCTP directly. The SS7 family of protocol was known for its high availability. The availability achieved what tied to error propagation method across the layers of protocol stack from Layer 1 to Layer 7. Such error propagation methods are not feasible in IP based protocol stack since many of the layers were designed independently and independent of applications.
The majority of early TCP/IP based communication did not involve large number of users connecting through a single association between nodes. For example even a single browser on a user computer may open several TCP connections with server(s) of the web objects. In telecommunication networks, it is rather common the association between two nodes carries communication for several thousand users. For example the SCTP based S1 interface between or Evolved Universal Terrestrial Across Network Node B (eNodeB) or Evolved Node B (eNB) base station and Mobility Management Entity (MME) carries signaling for all users connecting through that eNB. If the SCTP link were to fail, all users under that eNB will be unable to get cellular service.
Another consideration that is applicable to protocols in large networks is the scale of usage, i.e., how can be events or traffic be scaled up by utilizing more processing nodes that are connected by high capacity links. Thus load balancing and high availability consideration both put requirement on the underlying protocol implementation.
The SCTP is designed to be a host based protocol meaning there is only one SCTP association between two IP nodes. This is different than TCP where multiple TCP connections can exist between applications on two hosts. This aspect of SCTP has implication on both resilience as well as scalability. In a Long Term Evolution (LTE) network the MME keeps the mobility context for each attached user.